1. Field of the Invention
The present invention relates to the field of wireless communication, and more particularly, to a time division multiplexed communication system using the DECT protocol with spread-spectrum modulation.
2. Background
Communication between remote devices generally requires a communication protocol which enables devices to resolve which of them may transmit at any given time and which of them is designated to receive at any given time. One such protocol which is known in the art is the Digital European Cordless Telecommunications System (DECT) protocol developed by the European Telecommunications Standards Institute (ETSI). The DECT protocol includes provision for time division multiplexing between a base station and a mobile users, with each time frame comprising a base transmission portion and a mobile transmission portion divided into time slots. During the base transmission portion, the base station transmits in each time slot to a mobile user, and during the mobile transmission portion each mobile user transmits in an assigned time slot back to the base station.
Further information regarding the DECT system may be found in publication ETR 015 (March 1991) published by ETSI.
The DECT protocol provides a known protocol for transmission and reception between remote devices and is particularly well suited to communication at high data rates. In addition to being a uniform standard in Europe, where it has been allocated a frequency band around the 1.88 to 1.9 GHz region, the DECT protocol has been tested, used and found to be robust, and there is a substantial installed base of devices and systems which utilize the DECT protocol in Europe.
It would therefore be advantageous to be able to make use of the DECT protocol in the United States. One problem, however, is that there is presently no electromagnetic spectrum specifically allocated to the DECT protocol in the United States. Users who wish to use the DECT protocol may attempt to communicate using a frequency band where other types of communication are licensed (and thus must be compatible with those other types of communication), or must use a frequency band allocated to unlicensed devices (e.g., the 902-928 MHz frequency band). Communication in an "unlicensed" band is subject to a number of problems, due in part to the shared usage of that part of the spectrum which is required by regulation. These problems may include interference from other communicating devices, low power allowed to transmitters, and sources of unpredictable loss, noise, and interference.
Certain other communication systems make use of a technology known as spread-spectrum communication, in which transmitted signals are spread across a frequency band which is wider than the bandwidth of the data being transmitted. In spread spectrum communication, a data signal is typically modulated with a pseudo-random chip code to generate a transmitted signal spread over a relatively wide bandwidth. The transmitted signal has a low spectral density and appears essentially as noise to those not knowing the chip code. Consequently, spread spectrum communication provides increased security of transmitted information and reduced interference with other sensitive radio equipment being used in the surrounding environment.
Due to the nature of the spread spectrum signal it is typically necessary at the receiver to despread the received spread spectrum signal to recover the original data. In one spread spectrum technique, for example, despreading of the spread spectrum signal is accomplished by correlating the received signal with a reference code matching the pseudo-noise code used in the transmitter to encode the data prior to transmission of the information. After initial correlation is achieved, it is generally necessary to maintain synchronization by tracking the incoming signal so as to keep it aligned with the local reference code. While spread spectrum communication has been implemented in a TDD environment (see, e.g., U.S. Pat. No. 5,455,822 issued Oct. 3, 1995), many existing spread spectrum systems would be unable to perform synchronization fast enough to effectuate communication between sender and user within the constraints of a time multiplexed environment.